Why do knowledge-based agents only add a sentence to the knowledge base when it is 100% sure the sentence is true?
Mostly because traditional "knowledge based systems" are based purely on deductive logic and that's just the way deduction works.  It only deals with what consequences must follow from the premises.  Traditionally these systems didn't deal with probabilistic knowledge or other less strict forms of reasoning, like abduction.  

That said, it is possible to build a knowledge based system that includes probabilistic reasoning, and other reasoning systems (like abduction). 

I'll add some more on that point when I have a little more time.